An application of an organic semiconductor to a photoelectric conversion element such as an organic thin-film solar cell, an organic/inorganic hybrid solar cell, a light-emitting element, and an optical sensor is expected. In particular, using a high molecular compound as an organic semiconductor material makes it possible to fabricate an active layer by a low-cost coating method. From a viewpoint of energy requirement and an emission reduction of CO2, a solar cell is expected as one of clean energies with a small environmental load, and a demand for this is rapidly increasing. Currently, a silicon-based solar cell is prevailing, but its efficiency is around 15% and it is difficult to reduce its cost. As a solar cell that can be fabricated at low cost, a CdTe solar cell has also been known, but since it uses Cd being a harmful element, it is liable to cause an environmental problem. Under such circumstances, the development of an organic thin-film solar cell and an organic/inorganic hybrid solar cell as a next-generation solar cell that costs low, has high energy conversion efficiency, and is harmless is increasingly expected.
There is a strong demand for improving power generation efficiency of the organic thin-film solar cell in order to put the organic thin-film solar cell into practical use. In order to improve the power generation efficiency, improving an open-circuit voltage (Voc) is important. A value of the open-circuit voltage of the organic thin-film solar cell greatly depends on the combination of an electron donor and an electron acceptor, and it is required to optimize materials used for these. It has been known that the open-circuit voltage of the organic thin-film solar cell correlates with a difference between an energy level of a highest occupied molecular orbit (HOMO) of a p-type material and an energy level of a lowest unoccupied molecular orbit of an n-type material. It is thought that, in an organic thin-film solar cell currently under development, fullerenes such as phenyl-C61-butyric acid methyl ester (PCBM) are most suitable as the n-type semiconductor material. An example of a generally used p-type semiconductor material is a conjugate high polymer of polythiophene such as poly (3-hexylthiophene-2,5-diyl) (P3HT).
The open-circuit voltage (Voc) of the organic thin-film solar cell in which PCBM and P3HT are combined is low such as about 0.6 V and is not necessarily satisfactory in view of practical application. A possible method to improve the value of the open-circuit voltage may be to lower the HOMO level of the p-type semiconductor material. In this case, however, a band gap of the p-type semiconductor widens, and light in a long wavelength range cannot be absorbed. That is, light absorption efficiency for a long wavelength side of a visible light range reduces and incident light cannot be effectively used. There is a drawback that, as a result, energy efficiency does not increase. The value of the open-circuit voltage and the absorption of light in the long wavelength range are often in a trade-off relation, and it is difficult to achieve both at a higher level.
As one attempt to improve the value of the open-circuit voltage of the organic thin-film solar cell, using, as the p-type semiconductor material, a polymer in which imide is ring-condensed with thiophene is under consideration. In the organic thin-film solar cell using, as the p-type semiconductor material, the polymer in which imide is ring-condensed with thiophene, the open-circuit voltage improves up to about 0.85 V, but power generation efficiency is 1% or less, and a further improvement is required. Because of these, there is a demand for a p-type semiconductor material that improves a light absorbing property in a long wavelength range while increasing the value of the open-circuit voltage of the organic thin-film solar cell. Further, improvement of a life property in addition to improvement of an open circuit voltage is required of the organic thin film solar cell. In order to improve the life of the organic thin film solar cell, active substances (a donor and an acceptor) excellent in light stability and heat stability are required.
Further, researches have recently been made on an organic/inorganic hybrid solar cell whose energy conversion efficiency is improved by using an organic/inorganic hybrid perovskite compound or an inorganic perovskite compound for a photoelectric conversion layer. In the organic/inorganic hybrid solar cell, polyarylamine or 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene(spiro-OMeTAD) is used as a hole transport layer. Further, in order to enhance conversion efficiency, a dopant such as t-butylpyridine (TBP) or bis(trifluoromethanesulfonyl)imidelithium (Li-TFSI) is used. However, since TBP is liquid and Li-TFSI is a hygroscopic substance, there occurs performance deterioration caused by diffusion or dissipation of TBP to the photoelectric conversion layer due to a temperature increase, by absorption of water molecules due to deliquescence of Li-TFSI, and so on. This is a cause to shorten the life of the organic/inorganic hybrid solar cell. It has been also proposed to use P3HT being a p-type material as the hole transport layer, but sufficient power generation efficiency cannot be obtained in this case.